Electronic amplifier



Feb. 17, 1953 1 W SMITH ELECTRONIC AMPLIFIER 2 SHEETS-SHEET l Filed March 5, 1949 INVENToR Feb. 17, 1953 J. w. SMITH 2,629,073

ELECTRONIC AMPLIFIER Filed Maron 5, 1949 2 SHEETS- SHEET 2 AC GRID BIAS VOLTAGE IN PHASE SIGNAL A.C. BIAS -I- SIGNAL A VOLTAGE oUT oF PHASE SIGNAL TOTAL EFFECTIVE BIAS VOLTAGE IN VEN TOR. JAMES W. SMITH A TTU/PNE Y Patented Feb. 17, 1953 ELECTRONIC AMPLIFIER James W. Smith, Minneapolis, Minn., assignor to Minneapclis-Honeyweil Regulator Company, Minneapolis, Minn., a corporation of Delaware Application March 5, 1949, Serial No. 79,774

(Cl. S18- 19) 17 Claims. l

This invention is concerned with electronic amplifiers, particularly that type of electronic amplifier having an output stage comprising a discriminator circuit wherein the voltages on the control electrodes in the tubes in the discriminator circuit are the same in magnitude and phase.

It is an object of this invention to devise apparatus incorporating a plurality of ampliers electrically connected to a single bridge network to receive `different signal voltages therefrom with said amplifiers operating in sequence as the balance point of the bridge is varied, said amplifiers being so constructed as to prevent excessive current flow through the bridge network. i*

to ground. The voltages impressed on the cath- A ode and plate of each individual tube are of the same phase.

Another object of the invention is to provide means for preventing the control electrode in the tube in the discriminator stage which is not conducting from drawing current, being driven negative, and driving the control electrode of the other tube in the discriminator circuit negative and holding the said other tube out of the conducting range. This is accomplished by placing a capacitor between each control electrode and separate connections to the anode of the tube in the previous stage. This necessitates a voltage on one control electrode passing through two capacitors before it gets to the other control electrode and effectively prevents any direct voltage from passing from one control electrode to the other. Two capacitors are needed in order that the voltage impressed on the two control electrodes be the same.

Another object of the invention is to provide means for preventing the control electrodes in the tubes in the discriminator stage from producing an alternating current loading on the previous stage in the amplier. This is accomplished by placing a resistor of appreciable magnitude between each control electrode in the discriminator stage and the anode of the tube in the previous stage to damp out any alternating current which might tend to flow from control elec- This is accom- 2 trode to cathode in the tubes in the discrimin-ator stage.

Another object of the invention is to provide means in the discriminator circuit for discriminating against out-of-phase signals. This is accomplished by the use of a pair of grid-current limiting resistors and a pair of capacitors in the input cir-cuit to the tubes in the discriminator circuit. The resistors determine the maximum positive voltage which the grids of the tubes can assume with respect to the cathodes and the capacitors charge up an amount suicient to subtract a voltage from the alternating signal and bias voltages to leave a voltage no greater at any time than the maximum permissible positive voltage on the grids. The capacitors discharge through the grid-current limiting resistors and, in doing so, combine with the alternating signal and bias voltages to produce the effective signal voltage on the grids. When the -alternating signal voltage is not in phase with the supply voltage the effective signal voltage due to this interaction will also not be in phase with the supply voltage and will result in discrimination against the effective signal voltage and thus against the altern-ating signa1 voltage.

These and other objects and advantages may be better understood by a study of the following detailed description of the invention in conjunction with the drawing in which Figure 1 is a schematic diagram embodying the various features of the invention.

Figure 2 is a graph showing how the effective bias voltage is obtained in the discriminator stage with no signal input voltage;

Figure 3 is a graph showing how the effective bias voltage is obtained in the discriminator stage with an in-phase signal input voltage;

Figure 4 is a graph showing how the effective bias voltage is obtained in the discriminator stage with an out-of-phase signal input voltage;

Figure 5 is a graph showing how the effective bias voltage is obtained in the discrimin-ator stage with a signal input voltage; and

Figure 6 is a graph showing how the effective bias voltage is obtained in the discriminator` stage with a 270 signal input voltage.

A bridge network I0 is shown to have two signal voltage outputs taken off wiper arms 40 and 3l to operate a pair of ampliers A and B. Operation of amplifier A energizes a motor ISI while operation of amplier B energizes a motor I8I. Motor ISI is connected to wiper arm 40 to drive it to the null point on the bridge and motor I8I is connected to Wiper arm 31 to drive it to the aeaaovs null point on the bridge. Unless either wiper arm 40 or wiper arm 31 is at the null point both amplifiers will have a signal input voltage to cause operation of both ampliiers Mechanical stops are provided for the motors, however, to prevent the wiper arms from being moved off the follow-up potentiometers 3| and 21.

Two separate amplifiers are shown in the drawing and are designated as A and B. These ampliers are identical and therefore ampliier E has been shown only in box form.

The electrical bridge l furnishes the signal voltage to the two amplifiers. The bridge Iii is energized from a transformer secondary I having a primary |2 which is connected through leads I3 and |4 to the power leads i5 and |6 leading to a suitable source of power (not shown). As shown in the drawing, bridge l0 is a temperature control bridge and is capable of producing a signal voltage for heating or cooling. The bridge l0 has resistors l1, 2i! and 2| connected in series across transformer secondary Resistor 2i is the thermostat resistor and varies in temperature. Resistor 25 has a movable tap 22 ltied to ground at 23 and is a temperature setting resistance. lThat is, the variable tap 22 may be moved in one direction or the other along resistance depending upon the temperature desired, in order that the bridge will produce a signal voltage sufficient to energize either arn- :i

pliiier A or B when the actual temperature varies by a given predetermined amount from, the desired temperature. Resistor I1 is to balance the left side of the bridge, that is to approximately equal the resistance of thermostat 2| to provide 3;

a range of satisfactory operating points for wiper arm 22 along resistor 20.

Also connected in series across transformer secondary are i-lXed resistor 25, Calibrating resistor 26, cooling potentiometer 21, gap resistor 30, heating potentiometer 3| and iixed resistor 32. Cooling potentiometer 21 and heating potentiometer 3| are follow-up or rebalancing potentiometers and have throttling resistors 33 and 34, respectively, connected in parallel therewith. Calibrating resistor 23 has a movable wiper arm 35 connected to terminal 33 between calibrating resistor 23 and cooling potentiometer 21 which is movable across resistor 26 for adjustment purposes so that the proper amplifier will be energized at a given temperature difference from that temperature desired. Cooling potentiometer 21 has a Wiper arm 31 which passes over the potentiometer and is driven by a motor in a manner later to be described. The throttling resistor 33, placed in parallel with cooling potentiometer 21, is for the purpose of obtaining the throttling range desired, that is, the change in temperature which will cause the motor driving the wiping arm to drive the wiping arm from one end of the potentiometer winding to the other. Gap resistor 3|] is to prevent hunting at a changeover from operation of the wiper arm on the cooling potentiometer to the wiper1 arm on the heating potentiometer by providing a voltage range for a range of temperatures at which neither the heating nor cooling controls will be energized. The heating potentiometer 3| has a wiper arm 4i) which operates across the potentiometer winding in response to a motor in a manner later' tobe described. The throttling resistor 34 in parallel with heating potentiometer winding 3| serves the same purpose as does throttling resistor 33 with respect to heating potentiometer Winding 31. By the use of these throttling re- Ohms Resistor 17 1000 Resistor 20 30 Thermostat resistor 21 1000 Fixed resistor 25 1000 Calibrating resistor 26 30 Cooling potentiometer 27 135 Gap resistor 30 (for 1/2o to 1 gap) 1.3 Heating potentiometer 3l 135 Fixed resistor 32 1000 Throttling resistor 33 (for 5 throttling range) 6 r rthrottling resistor 34 (for 5 throttling range) 6' It is understood, of course, that the above values given are not the only ones that will work satisfactorily but that they are merely given as an example set of values.

Wiper arm on heating potentiometer 3| is connected by means of lead 4| to grid 42 of tube 43 in amplifier A. Correspondingly, wiper arm 31 of cooling potentiometer 21 is connected through lead 4| to a similar circuit in ampliner B. Tube 43 has in addition to grid 42, a plate 44 and a cathode 45. Grid 42 is connected to cathode 45 through a resistor 43 while the cathode 45 is connected to ground 41 through a resistor 50. The resistor is unbypassed in order to provide an alternating voltage bias on the cathode 45 and limit the amount of grid current which may be drawn by grid 42 upon the controlling temperature being in the range to cause operation of amplier B, energization of motor ISI', and movement of wiper arm 21. Drawing of grid current by tube 43 would load bridge l0 and affect the magnitude of signal at the position where the bridge is most nearly balanced. This is particularly important where, as in the present arrangement, the bridge l0 is connected to a plurality of ampliiiers.

rllhe plate 44 of tube 43 is connected to conductor 52, capacitor 53, and conductor 54 to grid 56 of tube 51 which is part of the second stage of amplifier A. Tube 51 also has an anode 60 and a cathode 5|. The grid 56 of tube 51 is connected to ground 52 through resistor 63. The cathode 6| is connected to a ground 54 through a resistor 55 which is bypassed by capacitor 06.

The plate or tube 51 is connected through lead 10, capacitor 1| and lead 12 to a grid 14 of a tube 15 comprising the third stage of ampliner A. Tube 15 has in addition to the control electrode or grid 14 a plate 16 and a cathode 11, The grid 14 is connected to a ground 80 through resistor 8| while the cathode 11 is connected to a ground 32 through an unbypassed resistor 83. The purpose of the unbypassed cathode resistor 83 in the third stage of the amplifier is to obtain a symmetrical square wave output from the third stage and to thus obtain proper operation of the succeeding amplifier stages.

A direct current plate supply for the rst three stages is obtained from a rectifier tube 92 and a transformer secondary 84 of a transformer 88 having a primary 85 connected `to power leads I5 and I6 through leads 36 and 81. One terminal of transformer secondary 84 is connected to ground at 95 while the other terminal is connected to plate di of the rectier tube 62 through lead 93. Tube 92 also has a grid 94 and a cathode 95. Because the tube used is a high mu tube, it is necessary to connect the grid 94 to the plate 9| through a lead 96 in order that the tube will operate as a rectier. The cathode 95 of the rectifier tube 62 is connected through lead 91,

a resistor |66, lead |D|, a resistor |62, lead |63,

resistor |64 and lead 5| to the plate 44 of tube 43 comprising the first stage oi' ampliiier A. A capacitor |05 is connected to lead |63 through a lead |56 and is connected to ground |61 through a lead I6 and is used as a lter to prevent ripple in the supply voltage from reaching the plate 44 of tube 43 and plate 65 of tube 51.

The cathode 95 of tube 92 is also connected to plate 60 of tube 51 in the second stage through lead 91, resistor |66, lead |51, resistor |62, leads |03 and resistor ||2 and lead 61. A iilter capacitor ||3 is connected from lead lill through lead ||4 and is tied to ground ||5 through lead ||6 to prevent ripple in the plate supply voltage from reaching the anode 16 of tube 15.

The greatest amount of filtering is obtained for the preliminary stages. The slight ripple which remains in the Supply voltage upon reaching the plate of the rst stage is amplified and reversed in phase and applied to the input circuit of the second stage. Because of this reversal in phase the ripple from the first stage applied to the second stage will be of opposite phase to the ripple on the plate of the second stage and the two ripple voltages will tend to cancel each other a' out. This is known as a hum bucking action.

Cathode 65 of tube 92 is also connected through lead 61, resistor |56, leads and ||1, resistor |24 and lead l2! -to anode 15 of tube 15 to furnish the plate supply voltage for the third stage. The output circuit of tube is connected to the input of the discriminator stage which comprises tubes |23 and |32. The connections are such that the voltage applied to the input circuits of tubes |23 and |32 are in phase. These connections will now be traced.

Anode 15 ci `tube 15 is connected to grid |22 of the discriminator tube |23 through a lead |24, a capacitor |25 and a resistor |21. Anode 16 is also connected to grid |3| of the discriminator tube |32 through leads |24 and |33, capacitor |34 and resistor |36.

Anode 16 is further tied to ground |46 through leads |24 and |4|, capacitor |42 and lead |43. The capacitor |42 acts as a short circuit to ground for high frequency.

rTube 23 has, in addition to grid |22, a plate |44 and a cathode |45. Tube |32 has, in addition to grid |3|, a plate |46 and a cathode |41. Grid |22 of tube 23 is tied to ground 56 through resistors |21 and |5|, and lead |53. Grid |3| of tube |32 is tiedvto ground |59 through resistors |36 and |54 and lead |53. An alternating power source for the plate |4|i oi tube |23 is supplied from transformer secondary of transformer 68 through lead |56. Plate |44 of tube |23 is further connected to a load circuit and ground through lead |56, transformer secondary |55, lead |51, relay coil winding |64 having a capacitor 6| in parallel therewith, lead |62 and ground |63.

consisting of lead Cathode |45 of tube |23 is connected to ground |64 through lead |65, one half of transformer secondary winding of transformer 88, and center tap |61 of transformer secondary winding |66 vto ground |64.

Anode |46of tube |32 receives plate supply Voltage from transformer secondary |1| of transformer 88 through lead |12 Anode |46 is further connected to a load and ground through a circuit |12, transformer secondary winding |1|, lead |13, relay winding |14 having capacitor |15 in parallel therewith, lead |16 and ground |11.

Cathode |41 of tube |32 is connected to ground |54 through lead |65, one half of transformer secondary winding |55, and the center tap |61.

The polarities of the transformer secondaries |55, |65, and |1| are such that when transformer secondary |55 drives plate |45 oi tube |23 positive with respect to ground, transformer secondary |56 drives cathode oi tube |33 positive with respect to ground. in the same fashion when plate |56 of tube |52 is positive with respect to ground, cathode |41 of the same tube is also positive with respect to ground. Thus it can be seen that transformer secondary |66 places an alternating voltage bias on cathodes |55 and |41 which necessitates a more positive signal voltage on the grid of the respective tubes to cause either one tube or the other to conduct when the plate is positive than would be the oase were there no bias present. This method of placing a bias on the cathodes of the two tubes in the discriminator circuit composed of tubes |25 and |32 niales it possible to use a single transformer secondary which has been center-tapped to ground and reduces the number oi parts or components hitherto considered necessary to bias voltages on the cathodes of tubes in discriminator circuits.

The capacitors |25 and |35 have been placed into the circuit so that the path from grid |22 of tube |23 to grid 53| ci tube |33 will go from grid |22, through resistor |21, capacitor 25, lead |33, and capacitor SEK-f, resistor to grid |3|. It is necessary to separate the grids in this manner so that, for example, when tube |23 would normally be in a condition to conduct, that is, with plate |45 having a positive potential with respect to the cathode |45 and a positive signal appearing on grid |22 and plate |45 of tube |32 being negative with respect to cathode |41, grid |3| of tube |32 will not have a tendency to draw current from cathode |41, drive grid |3| negative and impress that potential on grid |22 of tube |23 and so prevent tube |23 from conducting. If the voltage on grid |3| oi tube |32 could be impressed on grid |22 of tube |23 the action of tube |32 would be that of a D. C. restorer. The action of the two capacitors and is to stop any direct current iiow and thus prevent a tube whose plate is negative from acting as a D. C. restorer on the other tube which would normally be in the conducting range.

While the capacitors |25 and |34 stop the direct current which might new from either of the grids in tubes |23 and |32 they do not have any effect on the alternating current, which if it reached any magnitude, would have a deleterious effect on the operation of the third stage, which comprises tube 15. In order to reduce any alterhating current tendencies from grids |22 and i3! to insignificant limits grid limiting resistors |21 and |36 have been placed in the circuits of grids |22 and I3 respectively.

Resistors |5| and |54 operate to place bias l voltages on grids |25. and i3| respectively to stabilize the operation of tubes |23 and |32.

A motor |8| is shown having an armature |82 and field windings |83 and |84. Field winding |83 is connected in series to a capacitor |85 while eld winding Hifi is in parallel with capacitors |86 and |81. The junction of capacitors |85 and |81 is tied to a center tap |90 on field winding |84.

Field winding |33 is energized from power leads l and Iii as follows: power lead I5, leads |3 and lill, field winding |03, capacitor |35, leads |92 and I, and power lead lil. Thus it can be seen that field winding 83 is constantly enel'- gized.

The upper half of field winding |85 is energized from power lead i5 through leadsl I3 and |93, center tap |90 of nel@ winding |84, the upper half of held winding |851, movable relay con tact |94, fixed relay contact i355, and leads |96 and I4 to power lead iii. Movable relay contact |94 is moved in response to energization of relay winding lisl which is energized upon operation of tube |23.

The other half of iield winding I8@ is enersized from power lead iii through leads i3 and |93, center tap |90 of held winding |84, the lower half of iield winding |34, lead |91, movable relay contact 290, fixed relay contact and leads 202 and I4 to power lead I6. Movable relay ccntact 200 operates to make Contact with xed con tact 20| upon energization of relay winding |1i which is energized upon operation of tube E32.

When in a normal unenergized position, relay contacts |94 and |95 and E00 and 253! are open and eld winding |311 is unenergized. If both relay contacts |945 and lsb and and 20| were closed the current through the two halves of field winding 84 would cancel each other out and the armature |32 would still not rotate Because the movable contacts |94 and 200 close contact with xed contacts |9| and 20| respectively in accordance with operation oi tubes |23 and |31 respectively, it is not possible for both sets of contacts to close at the same time, due to the operation of tubes |23 and |32 previously explained.

Motor armature 282 is connected through mechanical linkage i503 to wiper arrn il@ on heating potentiometer 3| to move wiper arm /i in one direction or the other depending on the direction of energization of iield winding lila which causes selective rotation of armature |82.

One set of values which will operate satisfac torily in this circuit is as follows:

Tube t3, EL lZSLTGT Tube 15, .E lZSL-"l Tube |23., |32 2G50 Voltage across secoue. ie.. 250 volts Voltage across secondaries |1| 225 volts Voltage across secondary Einw. l0 volts It is understood of course, that the values given above are only for sake of example and do not limit my invention as various changes may easily be made by one skilled in the art.

Operation For the sake ci the description ci the operation, let it be assumed that the desired temperature is 70 F., and that wiper arm 22 has been adjusted to such a position on resistor 20 that 70 no unbalance voltage exists between wiper arms 22 and Q0 with wiper arm it at the lower end of resistor 3| and wiper arm 3l at the upper end of resistor 21. This position is shown in the drawing. Also let the operation be oonsidered during the half-cycle in which power lead lli is positive with respect to power lead I6. This results in the upper end of transformer Cccondary winding il being negative with repeet to its lower end, as is true with secondary ldings Sli, |60 and l'll. Now let it be ssurned that the region in which thermostat l is placed is suddenly cooled, decreasing the resistance of thermostat 2|. This will result in smaller resistance between wiper arm 22 and .e upper enel of transformer 1| than there was ien the temperature was at 100 F. The re- -ance between wiper arm lli] and the upper of transformer secondary remains the however, and thus wiper arm #i0 is driven positive with respect to wiper arm 22 and thus i1 respect to ground. This results in a signal passing through a positive half cycle being placed on grid 42 ci tube i3 causes tube lli to conduct. This alternating signal voltich has been impressed on gri-d 42 will be ed through tubes 4S, 51 and 'iii ancl the resulting output voltage from tube l5 which will. 'be passing through a negative half cycle will be impressed on grids 22 and il oi tubes E23 and its, respectively. At this haii ci the cycle the upper end of the transformer secondaries and 'ill are negative, it will be recalled, and thus plate ist of tube |23 is negative with ren spect to cathode M5 and the tub-e |23 will not conduct. Plate Ult of tube will oe positive with respect to cathode |131, however, but because of the negative signal voltage on grid |3|, tube |32 will not conduct either.

On the next half cycle the upper ends of transformer secondaries li, 0d, |55, li and 11| be positive with respect to their lower ends and so wiper arm :l0 will be negative with respect to arm 22. This results in grid 42 of tube being negative, plate @il of tube e3 thus of tube being positive, and plate of tube il grid 14 of tube being negative cause plate l0 of tube to be positive. This to la results in a signal voltage passing through a positive half cycle being placed on the grids |22 and lill of the discriminator tubes 23 and |32.

With the plate will and grid i222 of tube |23 both positive with respect to cathode |45 this tube will conduct and energize relay winding 5552i to close contacts 59:5 and |055 and energize upper half oi held winding 68d or motor |8l. causes armature |82 to rotate and move wiper arm @il on heating potentiometer 3| upward along the potentiometer until wiper arm and wiper arm 22 are again at the saine potential which will cause the signal voltage to grid i2 of tube #i3 to be reduced so that neither ci' tubes E9 and lili will conduct.

If the space in which thermostat 2| is placed now begins to heat up, the amplifier will operate cycle on grid 42 9 on the half cycle when the transformer secondaries Il, |55, |03 and |1| have their upper ends negative with respect to their lower ends. On this half cycle wiper arm 40 will be negative with respect to wiper arm 22 and thus place a signal voltage passing through a negative half of tube 43. This signal voltage will be amplified through the iirst three amplifying stages and will be impressed as a signal voltage passing through a positivehalf cycle on grids |22 and |3| of tubes |23 and |32 respectively.

Plate |43 of .tube |32 will now be positive with respect to cathode |41 andv tube |32 will conduct, energizing relay winding |14 and closing contacts 200 and 253| to energize the lower half of held winding |84 and cause armature |82 to rotate in the opposite direction to that previously described and move wiper arm 40 down along heating potentiometer 3| until the potential across wiper arms 40 andv 22 is zero and thus reducing the signal voltage on grid 42 of tube 43. When this signal voltage has been wiped out neither of the two tubes |23 and |32 in the discriminator circuit will conduct and thus neither relay winding |60 nor relay winding |14 will be energized and relay contacts |94 and |35 and 200 and 20| will be open and motor armature |82 will not be energized.

During the entire operation described so far, the'potential of wiper 31 has not been the same as that of wiper 22. in which the transformer secondaries I, |55, |62 and |1| have their upper ends negative with respect to their lower ends, the wiper 31 has been positive with respect to wiper 22. As will be clear from the foregoing explanation, the effect of this,

Thus during the half cycle since amplifier B and motor |8| are the same as ampliiier A and motor |8|, will be to tend to cause motor It i to move wiper arm 31 upwardly. Due to the fact that the wiper arm 31 is at the extreme upward range of its movement, however, this will be prevented either by a stop or a limit switch or some other suitable means for limiting movement of wiper arm 31. Thus, all the time that the temperature of the space in which thermostat 2| is placed is within the range of temperatures at which movement of the slider 4i! occurs, the motor ISI will remain stationary with the wiper 31 in its uppermost position. All this time, a signal is being impressed upon amplilier B. Due to the effect, however', of the cathode biasing resistor of amplifier B corresponding to the resistor 52 in amplifier A, the amount of grid current will be held to a minimum to prevent an undue effect upon the bridge. Thus, the cathode resistor of the amplifiers of the present invention is of particular importance with an arrangement such `as I have disclosed in which a plurality of rebalancing systems will be associated with a single bridge,

if the space in which thermostat 2| is placed rises a predetermined amount above the 70 F., represented by resistor 30, wiper arm 31 will be operated on and will control the energization oi amplifier B in the same manner as just described for amplifier A. That is, a rise in the temperature increases the resistance of thermostat 2i so that, upon the half cycle that the upper end of transformer secondary is positive with respect to the lower end, wiper arm 31, moving along cooling potentiometer 21, is positive with respect to wiper arm 22. As stated previously, amplifier B is identical with amplifier A and, therefore, because it has been seen that with wiper arm di? positive with respect to Wiper arm 22 there will be lno energization of motor |8| it follows that with l@ wiper arm 31 positive with respect to wiper arm 22 there will be no energization of motor I8 However, on the following half cycle amplifier B will be energized, just as is amplifier A, to energize motor ISI which will move wiper arm 31 down along potentiometer 21 until the voltage difference between wiper arms 31 and 22 is insufiicient to energize the discriminator stage in amplifier B.

Upon the temperature warming up and approaching 70 F., the wiper arm 31 will be negative with respect to wiper arm 22 on the half cycle that the upper end of transformer secondary is posiive with respect to the lower end and the energization of amplifier B will be reversed to that just described and cause motor I8 to move wiper arm 31 upwardly along potentiometer 21 until the voltage diiierence between wiper arms 31 and 22 is insufficient to energize the discriminator stage in amplifier B.

The motor armature |82 may also operate a damper or other device to control the temperature of the region in which thermostat 2| is placed in the direction called for by the bridge to bring Y the temperature back to l'10 F.

A brief description will now be given of the curves shown in Figures 2-6 which show the manner in which this circuit discriminates against out-of -phase signals. Let it be assumed that tube |23 of Figure 1 is the tube concerning which these curves have been drawn.

ln Figures 2 to 6, the curve 2li) indicates the plate voltage on plate |44 of tube |23 and is used as the reference voltage. The alternating grid i bias voltage which also remains constant with respect to the plate voltage has been designated as curve 2| and, as can be seen, is out-of-phase with the plate voltage. This voltage it is remembored comes from the upper half of transformer i secondary winding |66 and is the alternating grid bias voltage with which this invention is concerned. Curve 2|2 is the incoming signal voltage and is not seen in Figure 2 because this set of curves is for the no-signal input voltage condition. Curve 2I3 is the alternating component of the grid voltage due to the upper half of transformer secondary winding |63 and the incoming signal voltage added together. In a nosignal voltage condition assumed in Figure 2, this curve is identical with curve 2|| and thus, for the purpose of avoiding confusion, is not shown in the gure. Curve 2 I4 is the direct current bias voltage which is due to the algebraic sum of the alternating bias voltage plus the incoming signal voltage. rIhis direct current bias voltage results from grid rectification of tube |23 when grid |22 is positive with respect to cathode Mii. The grid current limiting resistor |21 operates to prevent the potential on the grid |22 from rising above a predetermined value of positive potential with respect to acthode |45. This grid current limiting property of resistor |21, with the similar action of resistor |32, operates to isolate the tubes |23 and |32 from each other in preventing operation of one of the tubes from aiiecting the potentials on the electrodes of the other tube. Adding curve 2|3 (in Figure 2 identical with curve 2|| and thus not shown) and curve 2id results in the total eective bias voltage cri the grid |22 of the tube |23. This curve is shown as curve 215. In the family of curves shown in Figure 2, it is seen that curve 2|5, the total efiective bias voltage, is out of phase with respect to the voltage on the plate which is curve 2id. It can be seen in Figure 2 that curve 2|5 never rises above a predetermined value above the axis. The direct current bias voltage charges up on capacitor 125 and is then discharged through resistor ibi. The values of capacitor i215 and resistor 151 determine the time constant which regulates the speed with which capacitor 25 charges and discharges. Because the total effective bias voltage is sufliciently negative during the positive half cycle of the plate voltage, tube 123 Will not conduct at any time under nosignal conditions.

In Figure 3 the in-phase signal condition is shown and to simplify the drawing the signal voltage from the previous stage is shown yto have the same magnitude as the alternating grid bias voltage continually impressed upon the tube Adding together curves 21 1 and 2l2 produces the curve 213 which in this case iles directly along the axis. As curve 211i depends upon the magnitude of curve 213 or in other Words as the direct bias voltage depends upon the total alternating bias voltage, curve 214 for the direct bias voltis non-existent in the ycondition shown in Figure 3 and thus is not shown. As curve the total eiective bias voltage, is dependent upon the total alternating bias voltage plus the direct bias voltage the curve 215 also lies along the axis and has a constant zero value under the conditions shown. Therefore, as indicated by the shading, tube 123 conducts during the entire positive half cycle of the plate voltage on plate 144 of the tube.

Figure e shows the condition Where an out-ofphase voltage is impressed upon grid 1271.. from the previous stage. In this stage the curve 2i i, the alternating grid bias voltage, and curve 2i?, the incoming signal voltage, are superimposed and appear as one curve. Curve 2id is shown as the result of the addition of curves 21a 242 and thus indicates the total effective alternating bias voltage. As explained in Figure 2, the curve 2M which is the direct current bias voltage resulting from charging of the capacitor 125 through resistor 151 and due to the effect of the current limiting resistor 12.?1 is shown as opposing the total alternating bias voltage by a suiiicient amount to prevent the total effective bias voltage on the grid 122 of the tube from exceeding a predetermined given positive value. The addition of curves 213 and 214 then produce the curve 215 which, because it is negative with respect to the positive plate voltage on the positive half cycle of the plate voltage, prevents the tube from conducting during any part of the cycle.

In Figure 5, the condition is shown for a 90 signal voltage. ln this case again, the plate voltage Zio and the alternating grid bias voltage 2li are not changed. As can be seen in the drawing, however, the curve 212 which is the incoming alternatin't signal voltage has been advanced by 90 and the addition of these two curves, 21 l and 212, `produce the total altern-ating bias voltage curve 213. Again, because the grid current limiting resistor 12'1 prevents the potential on grid 122 from rising above a given predetermined positive value, the curve 211i is of such value as to prevent the total effective bias from rising above that given predetermined value. The resulting curve 2lb again is the result of the addition of curves 213 and 2 itl and as can be seen in these drawings, has decreased to a suiiiciently small negative value near the end of the Apositive half cycle of the plate voltage that the tube conducts for a very short period of time. This small period of con- Cil Ill)

l2 duction, however, is insufficient to cause operation of einotor 381,

in Aigure the 270 signal voltage condition is shown. in this case again, the plate voltage curvo 2id and the alternating grid bias voltage 21 1 due to the upper halt of transformer secondary Winding remain-.s the same. It is seen, however, that the incoming signal voltage 212 has been displaced to 90 behind the alternating grid bias voltage and the addition of curves 212 and lill produce the resultant total alternating bias voltage curve 213. Again, curve 214 is dependent in magnitude upon the magnitud e of the total alternating; bias voltage curve to prevent the voltage on the grid 122 from rising above a predetermined positive vaiu-e as has been before The addition -of curves 214 and I213 prostated. duce the total effective bias voltage curve 215 which, as in the `drawing of Figure 6, is sufi` ciently small in a negative direction to permit conduction during; the first part of the cycle when the `plate voltage is positive. This small vamount ci conduction is, however, insumcient to cause operation of motor 181i.

lt is thus demonstrate-ol by the curves shown for these various conditions the -manner in which the con-duction oi' the tube will vary with change in of the input signal voltage. It thus should be readily obvious that for a. signal which is, i-cr example, only 660 out of phase, the tube 123 will conduct for a greater part of the cycle than it doe; for 90 signal though it will not conduct over the entire half cycle. Likewise, if the signal should, for example, be 300 ou-t of phase the tube will conduct over a greater portion of the niet part of the cycle when the'piatc voltage is positive though it will not conduct over the entire positive half cycle. Also, if the signal should be progressively greater than 90 out of phase or progressively less than 270 out of phase, the conduction of the tube Will'be less and less until eventually the tube will not conduct at all.

While I have shown a single circuit embodying the various features of my invention it will be apparent to those skilled in the art that 4modications be `made without departing vfrom the s ,ope or spirit of the invention and that accordingly l consider my invention limited only to the scope of the claims yappended hereto.

claim as vmy invention:

l. l'n combination: signaling means comprisymg a bridge having a yplurality of output signal voltages, each of which differs in value from the remainder; rebalancing means for independently reducing any one of said voltages to zero; a plurality of electronic ampliers; means connecting said amplifiers with said bridgesuch that each amplier is affected by a separate output signal voltage from said bridge; a separate motor energized by each of said ampliiers; mea-ns connecting said motors to said rebalancing means; means enabling only one of said rebaiancing means to be rebalanced at a given time; and bias lmeans in each of said ampliiiers to limit the grid current in said amplifiers and reduce loadingt of said bridge by said amplifiers.

2. ln combination: a controlling condition: a plurality of signal voltages resulting from said controlling con i n; means energized from said signal voltages to reduce any one of said voltar-res to zero, said lmeans comprising an amplifier to be operated by each of said voltages, a motor to be energized by each of said amplifiers,

a connection between each motor and an input circuit to the amplifier energizing that motor by which energization of each mot-cr tends to reduce the signal voitag-e to that amplifier to zero, limi-ting devices to prevent more than one voltage from being reduced to zero at a time, and biasing means in each or" said ampliers to lim-it the grid current `drawn by said amplifiers and reduce loading of said lcontrolling condition by said amplifiers.

3. in combination: an electrical bridge signa ing means having a .plurality of signal voltage out-puts each differing in value from the remainder; a plurality of amplifiers each having a plurality of stages; means connecting said amplifiers to said bridge such that each amplier is affected by a separate output voltage from said bridge; bias means lin the rst stage oi each amplifier such as to limit the grirl Acurrent `drawn by the first stage and reduce lpossible loa-ding of the bridge; and a further stage in each -amplii'ier having a selective output voltage dependent upon the direction of the signal voltage causing operation of the ampliiier.

4. In combination: an electrical bridge signaling means having a plurality of signal voltage outputs, each diiering in value from the remaining signal voltages; a plurality of ampliers each having a plurality of stages; means connecting each of said ampliers to separate bridge outputs 1f" ing of the bridge; a further stage in each of said amplifiers having a rst and a second electron tube, each having a plate, a cathode and a control electrode, comprising a discriminator stage; means in each of said amplifiers impressing a signal voltage, dependent upon the signal f voltage from said bridge, of the same phase and magnitude on the control electrodes in said discriminator stage; means electrically separating the control electrodes in said discriminator stages to prevent one control electrode from affecting operation of the tube containing the other conof the tubes in the discriminator stages, the phase of the voltage on one of the plates in each discriminator stage being opposite in phase to the voltage on the other plate in the discriminator stage; and means applying an alternating bias voltage on the cathodes of the tubes in the I, discriminator stages, the bias voltage placed on each of the cathodes being of the same phase as the voltage on the plate of that tube.

5. A first and a second electron tube, each having a plate, a cathode and a control electrode, comprising a discriminator stage; a means for applying a single signal voltage to both of said control electrodes; means electrically separating said control electrodes to prevent the operation of one of said tubes from affecting the operation of the other of said tubes, said means comprising resistors and capacitors in series; and means for applying an alternating bias voltage to said cathodes to prevent simultaneous operation of both tubes, said bias voltage on one cathode being of the opposite phase of the bias voltage on the other cathode, said means charging said capacitors in such a manner as to produce a negative bias on said control electrodes.

6. A rst and a second electron tube, each having a plate, a cathode and a control electrode, comprising a discriminator stage; means applying the same signal voltage from a signal voltage source to both of said control electrodes; a source of alternating bias voltage connected to said cathodes; and means electrically separating said control electrodes to prevent the control elec- -trode of one tube from drawing current and holding both control electrodes negative, said means comprising a capacitor connected between said control electrodes, said capacitor being charged by means including said source of alternating bias voltage.

7. A first and a second electron tube, each having a plate, a cathode and a control electrode, comprising a discriminator stage; means applying the same signal voltage from a signal voltage source to both of said control electrodes; means electrically separating said control electrodes to prevent the control electrode of one tube from drawing current and holding both control electrodes negative, said means comprising a capacitor connected between said control electrodes; and means for applying an alternating bias voltage to said cathodes to prevent simultaneous operation of said tubes, the bias voltage on one of said cathodes being of the opposite phase of the bias voltage on the other of said cathodes, said alternating bias voltage charging said capacitor in such a manner as to provide a negative bias voltage on said control electrodes when the magnitude of said signal voltage is less than the magnitude of said alternating bias voltage.

8. A rst and a second electron tube, each having a plate, a cathode and a control electrode, comprising a discriminator stage; a source of alternating signal voltage; means applying an alternating signal voltage of the same phase and f; magnitude from said source to both of said control electrodes through separate capacitors; means electrically separating said control electrodes to prevent the potential on the contrci electrode of one tube from affecting operation of the other tube, said means comprising said separate capacitors connected between said control electrodes; means for preventing loading of said signal voltage source by said control electrodes, said means comprising a resistor between said control electrodes and said source; and means for applying an alternating bias voltage to said cathodes to prevent simultaneous operation of said tubes, the bias voltage on one of said cathodes being of the opposite phase of the bias voltage on the other of said cathodes, said alternating bias voltage charging said capacitor in such, a manner as to provide a negative bias on said control electrodes.

9. A rst and a second electron tube, each having a plate, a cathode and a control electrode, comprising a discriminator stage; a source of alternating signal voltage; means applying an alternating signal voltage of the same phase and magnitude from said source to both of said control electrodes; means applying an alternating plate supply voltage to the plates of said tubes, the voltage on one of said plates being opposite in phase to the voltage on the other of said plates; and means applying an alternating bias voltage cn the cathodes of said tubes, the bias voltage placed on each of said cathodes being of the same phase as the voltage on the plate of that tube.

10. A rst and a second electron tube, each having a plate, a cathode and a control electrode,

comprising a discriminator stage; a source of alternating signal voltage; means applying an alternating signal voltage of the same phase and magnitude from said source to both of said conelectrodes: means eicctreally separating control electrodes to prevent the control electrode in one tube from affecting operation of the other tube, said means comprising a capacitor between said control electrodes; means for preventing loading of said signal voltage source by said conl trol electrodes, said means comprising a resistor between said control electrodes and said source; means applying an alternating plate supply voltage to the plates of said tubes, the voltage on one of said plates being opposite in phase to the voltage on the other of said plates; and means applying an alternating bias voltage on the cathodes of said tubes, the bias voltage placed on each of said cathodes being or the same phase as the voltage on the plate of that tube.

11. In combination: a rst and a second electron tube each having an anode, a cathode and a control electrode; means for applying a rst alternating' voltage ci a nrst phase between the anode and cathode of said first tube; means for applying a second alternating voltage of opposite phase to said first voltage between the anode and cathode ci se' second tube; input circuit for said iirst tube including the control electrode c for said second tube including the control electrode and cathode of said second tube; a source of alternating signal. voltage; means for applying a single alternating voltage from said source beinput circuit :if

tween the control electrode and cathode of each f stage in each amplifier having a pair of electron tubes each having an anode, a cathode and a `control electrode; means for applying a first alternating voltage of a first phase between the anode and cathode of Said first tube; means for applying a second alternating voltage of opposite phase to said first voltage between the anode and cathode of said second tube; an input circuit for said iirst tube including the control electrode and cathode of said 'lirst tube; an input circuit for said second tube including the control electrode and cathode of said second tube; a source of alternating signal voltage; means for applying a single alternating voltage from said source between the control electrode and cathode of each tube; and means for causing said tubes to discriminate against input voltages which are not in phase with the supply voltages said means comprising a resistor in each input circuit to limit the extent to which the voltages on the control electrodes may be positive with respect to the cathodes, and a capacitor in each input circuit which charges up to the extent that the input voltage is more positive than the limited permissible positive voltage between the control electrode and cathode and then discharges t0 cause the control electrode in the same input circuit to have impressed thereon a voltage of greater negative magnitude with respect to the cathode than is the magnitude of the input voltage.

i3. In combination: an electrical bridge signaling means having a plurality oi signal voltage 0utputs, each diiiering in value from the remaining signal voltages; a plurality of amplifiers each having a plurality of stages; means connecting each of said amplifiers to separate bridge outputs to provide the signal voltage inputs to said amplifiers; further means connecting each amplier to said bridge such that operation of said amplitrode and cathode and then discharges to cause AI the control electrode in the same input circuit to have impressed thereon a voltage of greater negative magnitude with respect to the cathode than is the magnitude of the input voltage, the capacitors further being effective to prevent the potential on the control electrode of one tube from aecting operation oi the other tube, the resistors further being eiective to prevent loading of said source by the potentials on said control electrodes; and means for applying an alterl nating bias voltage to said cathodes to prevent simultaneous operation of said tubes, the bias voltage on one of said cathodes being of the opposite phase of the bias voltage on the other said cathodes.

l2. In combination: an electrical bridge signaling means having a plurality of signal voltage outputs, each diiierinfr in value from the remaining l a irality of' amplifiers each stages; connecting fier due to the signal from said bridge to said amplifier tends to reduce the signal voltage to that amplifier to Zero; bias means in the first stage of each of said ampliliers such as to limit the grid current drawn by the first stage and reduce possible loading of the bridge; a further stage in each amplier having a pair of electron tubes each having an anode, a cathode and a control electrode; means for applying a first alternating voltage of a first phase between the tanode and cathode of said first tube; means for applying a second alternating voltage of opposite phase to said first voltage between the anode and cathode of said second tube; an input circuit for said nrst tube including the control electiode and cathode of said rst tube; an input circuit for said second tube including the control electrode and cathode of said second tube; a source of alternating signal voltage; means for applying a singlealternating voltage from said source between the control electrode and cathode ci each tube; and means for causing said tubes to discriminate against input voltages which are not in phase with the supply voltages, said means comprising a resistor in each input circuit to limit the extent to which the voltages on the control electrodes may be positive with respect to the cathodes, and a capacitor in each input circuit which charges up to the extent that the input voltage is more positive than the limited permissible positive voltage between the control electrode and cathode and then discharges to cause the control electrode in the saine input circuit to have impressed thereon a voltage of greater negative magnitude with respect to the cathode than is the magnitude of the input voltage, the capacitors further being eiective to prevent the potential on the control electrode of one tube from affecting operation of the other tube, the resistors furtherv being eiective to prevent loading of said source by the potentials on said control electrodes, and means for applying an alternating bias voltage to said cathodes to prevent simultaneous operation of said tubes, the bias voltage on one of said cathodes being of the opposite phase of the bias voltage on the other of said cathodes.

14. An electronic amplier circuit, comprising in combination; two electronic discharge devices each having a cathode, an anode, and a control electrode; a source of alternating` bias potential having two end terminals and a center tap thereon; circuit means connecting one of said end terminals to one of said cathodes and circuit means connecting the other of said end terminals to the other of said cathodes; two input circuits each comprising a resistor and a capacitor; circuit means connecting one of said resistors and one of said capacitors to one of said control electrodes; circuit means connecting the other of said resistors and other of said capacitors to the other of said control electrodes; a source of alternating signal potential; and circuit means connecting said source of signal potential to said capacitors, said signal potential selectively rendering one of said discharge devices conductive when the magnitude of said signal potential is greater than the magnitude of said alternating bias potential.

15. An electronic amplifier circuit comprising in combination; a discriminator circuit comprising at least two electronic discharge devices each having an anode, a control electrode, and a cathode; a source of alternating bias potential having two end terminals and a center tap thereon; circuit means connecting one of said cathodes to one of said end terminals; circuit means connecting the other of said cathodes to the other of said end terminals; two sources oi alternating potential; circuit means connecting one of said sources to one of said anodes and circuit means connecting the other of said sources to the other of said anodes; an input network comprising at least four resistors and two capacitors; circuit means connecting said resistors in a series circuit between said control electrodes and said two condensers in series shunting the two resistors in the center of said series circuit; a source of alternating signal potential, said potential having the same frequency of said alternating bias potential; and circuit means connecting said signal source to the common junction between said condensers so that said signal potential will selectively render one of said discharge devices conductive when the magnitude of said signal potential at least is equal to the magnitude of said bias potential.

16. An electronic amplier circuit, comprising in combination; an electronic discharge device having an anode, a cathode, and a control electrode; a source of alternating potential having two terminals; circuit means connecting one terminal of said source of potential to said anode;

a load device; a source oi alternating bias potential having two end terminals; circuit means connecting one of said end terminals to said cathode and the other of said end terminals through said load means to the other terminal of said source of alternating potential; further circuit means connecting said other of said end .terminals through a resistor to said control electrode; a capacitor; circuit means connecting said vcapacitor to said control electrode in such a manner that said alternating bias potential charges said capacitor thereby maintaining a negative bias on said control electrode; a source of alternating signal potential; and circuit means connecting said signal potential source to said capacitor, said source rendering said discharge device conductive when the magnitude of said signal potential exceeds the magnitude of said alternating bias potential.

17. An electronic amplifier circuit, comprising in combination; a discriminator stage comprising two electronic discharge devices each having an anode, a control electrode, and a cathode; two sources of alternating potential; circuit means connecting one or" said sources to one of said anodes and the other of said sources to the other of said anodes; an input circuit for each of said discharge devices comprising a resistor and a capacitor; circuit means connecting said input circuits to said control electrodes; a source of alternating bias potential for each of said devices, said source having two end terminals and a center tap thereon; circuit means connecting one of said end terminals to one of said cathodes and the other `of said end terminals to the other of said cathodes and the center tap to the input circuits oi said discharge devices in such a manner that said alternating bias potential in combination with said means connecting said source of bias potential to said input circuits eiectively renders a negative bias voltage on said control electrodes; a source of alternating signal potential; and circuit means connecting said signal potential to said input circuits in the same phase relationship, said signal potential selectively rendering one of said discharge devices conductive when the magnitude of said signal voltage at least equals the magnitude of the signal from said alternating bias potential.

JAMES W. SMITH.

REFERENCES CITED The following references are of record in the le of this patent:

' UNITED sTA'rns PATENTS Number Name Date 1,958,245 Mittag May 8, 1934 2,164,728 Wey July 4, 1939 2,288,600 Arndt July 7, 1942 2,399,695 Satterlee May 7, 1946 2,414,384 Moseley Jan. 14, 1947 2,426,711 Shaffer Sept. 2, 1947 2,434,822 Van Beuren Jan. 20, 1948 2,450,084 Emerson Sept. 28, 1948 `2,452,311 Markusen Oct. 26, 1948 

